All lens have some degree of distortion. Here are 3 common forms of lens distortion.

Barrel distortion

In barrel distortion, image magnification decreases with distance from the optical axis. The apparent effect is that of an image which has been mapped around sphere (or barrel). Fisheye lenses, which take hemispherical views, utilize this type of distortion as a way to map an infinitely wide object plane into a finite image area. In a zoom lens barrel distortion appears in the middle of the lens's focal length range and is worst at the wide-angle end of the range.

Pincushion distortion

In pincushion distortion, image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image, like a pincushion.

Mustache distortion

Is a mixture of both barrel and pincushion types, sometimes referred to as mustache distortion or complex distortion, is less common, but has been known to appear from time to time. It starts out as barrel distortion close to the image center and gradually turns into pincushion distortion towards the image periphery, making horizontal lines in the top half of the frame look like a handlebar mustache.